Generally, for such a hands-free system, a group of antennas is intended to detect the presence of the tag outside the vehicle and another inside the latter. If an authorized tag is detected outside the vehicle, the control and management system will allow access on board the vehicle without however allowing starting, whereas if this tag is inside the vehicle, starting of the engine will be able to occur.
To locate the tag, a signal is sent to a group of antennas which then carry out a corresponding transmission while no signal is sent to the other antennas so that they remain mute. The antennas used are LF (low frequency) antennas transmitting a magnetic field with a frequency usually lying around 125 kHz with a restricted range thus allowing good locating of the corresponding tag. The dialogs between the inside antennas and the tag are of a different kind from the dialogs between the outside antennas and tag. This enables the tag to know in advance the response expected by the vehicle. This response is generally effected by transmitting an RF (radio frequency) signal with a frequency generally lying in the vicinity of 433 MHz.
The inside antennas are generally regarded as reliable. The field transmitted by them does not generally spill over out of the cabin of the vehicle. On the other hand, the outside antennas generally transmit a signal inside the cabin of the vehicle too, generally limited to near the latter's doors. A tag is then regarded as being outside the vehicle if it responds to the outside antennas without however responding to the inside antennas.
The locating of a tag outside the vehicle is therefore based on a nonresponse of the tag to a request by the inside antennas. It may however happen that the tag responds to a request by the inside antennas but that this response is not received by the antennas management device. This problem may stem from several causes, such as for example the presence of glitches or the sending of an RF frame of another system using the same frequency band. This nonreceipt of the response by the vehicle antennas management system will be interpreted as a nonresponse to the request of the inside antennas and the tag will then be regarded as being outside the vehicle although it is in fact inside the latter. The management device will therefore give its agreement to execute an order to lock the vehicle although the user does not have his tag on him. Moreover, anybody will be able to unlock the vehicle with the tag still in the vehicle.